Investigations from our laboratory revealed that Nov, a member of the CCN family of proteins, is expressed by osteoblasts and its transcript levels are regulated by transforming growth factors (TGF) beta and bone morphogenetic proteins (BMP). CCN proteins have been shown to bind and interact with TGF beta, BMPs and Wnt, but the function of Nov in cells of the osteoblastic lineage is not known. Consequently, we explored the function of Nov in skeletal cells, in vitro and in vivo, and found that Nov binds BMPs and determines the cellular fate of undifferentiated stromal cells. Constitutive overexpression of Nov in ST-2 stromal cells resulted in impaired osteoblastic differentiation. Nov overexpression opposed biological effects of BMP-2 and Wnt 3 and impaired BMP-2 and Wnt signaling in ST-2 cells, indicating that Nov has BMP-2 and Wnt antagonistic activity, as it has been reported for classic BMP antagonists. In addition, transgenic mice overexpressing Nov under the control of the osteocalcin promoter exhibited osteopenia secondary to decreased bone formation. The aim of the proposed studies is to understand the function of Nov in bone in vivo and in vitro and define mechanisms involved in its inhibitory effects on osteoblastogenesis and bone formation. For this purpose, we have created transgenic mouse lines overexpressing Nov and created nov null mice. Our specific aims are: 1) To determine the function of Nov in vivo by transgenic overexpression of Nov under the control of the type I collagen or the osteocalcin promoter. The skeletal phenotype of transgenic mice will be compared to that of wild type mice and determined by histomorphometry, contact radiography, densitometry and micro CT scanning; 2) To determine the function of Nov in vivo by targeted gene deletion. The skeletal phenotype will be assessed with histomorphometric, radiological, and structural techniques; and 3) To determine the mechanism of action of Nov in vitro in stromal and osteoblastic cells overexpressing Nov and examining the behavior of nov null cells. The impact of Nov on the differentiation of cells of the osteoblastic lineage and mechanisms involved will be determined. These investigations should clarify the role of Nov in bone cell function, and its interactions with BMP and Wnt signaling pathways.